The art of printing typically includes rasterizing continuous tone master images, such as from transparencies, typically via half-toning. As is known in the art, half-toning is produced by superimposing a screen, having a known size, spatial frequency and angle, onto the continuous tone master image.
It is common to desire to reproduce the half-tone master on a reproduction system other than that used to produce it. For example, the half-tone master may have been produced for printing via an offset process and it is desired to reproduce it for printing via a gravure process. Alternatively, it may be desired to reproduce the half-tone master on a printing press with a different screen.
Since the screen used for the second printing method is typically different from the original screen, a Moire effect will arise if the original screen is not removed.
The process of removing the original screen before rescreening is known as descreening, or half-tone (HT) to continuous tone (CT) conversion, various elements of which are known in the art.
For example, descreening can be performed optically, as is described in U.S. Pat. Nos. 4,516,175 to Jung and 4,231,656 to Dickey et al. U.S. Pat. No. 4,516,175 describes an opto-electronic image scanning device with a scanning diaphragm having an aperture of variable size and shape for removing the original screen. The scanning spot produced by the aperture must be an integral multiple of the original screen size and the aperture must be aligned with the screen angle.
U.S. Pat. No. 4,231,656 utilizes an optical filter for removing the undesired screen frequency from a half-tone image and for modifying the amplitudes of the remaining frequencies in the frequency domain.
U.S. Pat. No. 4,403,258 to Balzan et al also performs optical descreening. The apparatus includes two or more concentric apertures which generate output signals according to the average tone variations scanned from the half-tone image in each of them. The signals are weighted electronically to produce the descreened image.
European Patent Publication 301,786 to Crosfield describes a digital filtering system for removing a screen from an half-tone image. The system includes apparatus for first scanning the half-tone image into a binary digital image, apparatus for averaging the binary image in order to reduce the number of pixels, and apparatus for descreening. The apparatus for descreening performs a Fast Fourier Transform (FFT) on a slice of the image, low pass filters the transformed image in the frequency domain, and finally, performs an Inverse FFT to obtain the descreened image. The method for descreening is described in detail in European Patent 195,563 also to Crosfield.
European Patent Publication 074,795 to Xerox describes an image scanning system which quickly scans a line and which determines whether or not to rescan the line slowly for processing by a half-tone processor. The half-tone processor is a digital filter and is preferably a low pass filter.
The following books and articles also discuss digital filtering and descreening type processes:
"A Survey of Electronic Techniques for Pictorial Image Reproduction" IEEE Transactions on Communications, Vol COM 29, No. 12,, December 1981, pp. 1898-1925;
Digital Image Processing, Gonzalez, et al, Addison Wesley, 1977, pp. 218-233;
Digital Signal Processing, Oppenheim and Schafer, Prentice-Hall, 1975, pp. 239-250;
Theory and Application of Digital Signal Processing, Rabiner and Gold, Prentice Hall, 1975; and
"On the Use of Windows for Harmonic Analysis with the Discrete Fourier Transform", Frederic J. Harris, Proceedings of the IEEE, Vol. 66, No. 1, Jan. 1978, pp. 51-84.